The invention relates to digital filters used for channel equalization and cancellation of multipath distortion in digital communications radio receivers, such as those employed in digital television receivers.
A known configuration of channel equalizer employs a precursor finite-impulse-response (FIR) digital filter followed by a postcursor infinite-impulse-response (IIR) digital filter. The postcursor filter comprises a digital subtractor receiving the IIR precursor filter response as minuend input signal, a quantizer for quantizing the difference output signal from the subtractor, and a feedback FIR digital filter responding to the quantizer output signal for supplying subtrahend input signal to the subtractor. The postcursor filter suppresses post-ghost signals arriving after the principal signal. The precursor filter is commonly called a xe2x80x9cfeed-forward FIR filterxe2x80x9d to distinguish it from the feedback FIR filter in the postcursor filter. The feed-forward FIR filter combines match filtering to reduce intersymbol interference, filtering to suppress pre-ghost signals arriving before the principal signal, and filtering to suppress artifacts otherwise arising in the postcursor filter. Clocking of the digital filters in the channel equalizer must be at a rate at least as high as symbol rate in order to satisfy the well-known Nyquist criterion for pulse reproduction without irreparable intersymbol interference (ISI) being introduced.
In a process known as xe2x80x9csynchronous equalizationxe2x80x9d the received signal is subjected to various delays that are multiples of the symbol interval and is summed with the delayed signals in a weighted summation procedure to suppress multipath distortion. Synchronous equalization has been employed in adaptive channel equalizers in which the feed-forward and feedback FIR filters are clocked at symbol rate. In such adaptive channel equalizers the coefficients of the feed-forward and feedback FIR filters are adjusted during operation by a process known as xe2x80x9cdecision feedbackxe2x80x9d. Error signal for the decision feedback method is generated by comparing the output signal from the quantizer with its input signal, both signals being clocked at symbol rate.
In a process known as xe2x80x9cfractional equalizationxe2x80x9d the received signal is subjected to various delays that are multiples of a specified fraction of the symbol interval and is summed with the delayed signals in a weighted summation procedure to suppress multipath distortion. Equalization at band edges is known to be much improved in a channel equalizer clocked at twice symbol rate, in which channel equalizer the received signal is subjected to various delays that are multiples of one-half of one symbol epoch. It is also known that substantially the same degree of improvement can be obtained with a channel equalizer filter with substantially fewer taps that is clocked at four-thirds symbol rate. In such channel equalizer the received signal is subjected to various delays that are multiples of three-quarters of one symbol epoch.
In over-the-air digital television, transmission channel characterization is subject to considerable change with time and adaptive coefficient equalization is a practical necessity for a DTV receiver to be commercially acceptable. There is a desire to employ decision feedback techniques for adjusting the coefficients in the feed-forward and feedback FIR filters in order to track changing multipath conditions. Fractional equalization is preferred in the adaptive channel equalizer so there is less criticality as to the timing of sampling in the component filters.
Since error signal for the decision feedback method is generated by comparing the output signal from the quantizer with its input signal, both signals being clocked at symbol rate, it has been proposed to employ synchronous equalization in the feedback FIR filter and to carry out fractional equalization entirely in the feed-forward FIR filter. Apparently, rate-reduction filtering or decimation of the FIR precursor filter response is performed to generate input signal for the IIR postcursor filter in this proposed scheme.
The inventors point out that the problem with this proposal is that the error signal generated by comparing the output signal from the quantizer with its input signal, both signals being clocked at symbol rate, has insufficient digital bandwidth for properly adjusting the feed-forward FIR filter coefficients by decision-feedback methods.
In a radio receiver for digital transmissions, an adaptive channel equalizer is clocked at a sampling rate higher than symbol rate by a factor k and employs decision feedback for adjusting the coefficients of its component filters. The channel equalizer response as supplied at the sampling rate k times symbol rate is decimated to symbol rate and quantized to generate a signal estimating the symbols transmitted to the receiver. The channel equalizer response as supplied at the sampling rate k times symbol rate is compared to the signal estimating the transmitted symbols as re-sampled to the sampling rate k times symbol rate, in order to develop decision-feedback error signal at the sampling rate k times symbol rate. This decision-feedback error signal has sufficient digital bandwidth to permit fractionally spaced equalization with all coefficients being optimally adjusted.
In preferred embodiments of the invention the adaptive channel equalizer employs a precursor finite-impulse-response (FIR) digital filter followed by a postcursor infinite-impulse-response (IIR) digital filter. The IIR precursor filter supplies fractionally spaced response at a sampling rate higher than symbol rate. The postcursor filter comprises a digital subtractor receiving IIR precursor filter response as minuend input signal, a rate-reduction filter responsive to fractionally spaced difference output signal from the subtractor for supplying a rate-reduction filter response clocked at symbol rate, a quantizer for quantizing the rate-reduction filter response, and a cascade connection of a feedback FIR digital filter and an interpolation filter. The cascade connection responds to the quantizer output signal for supplying subtrahend input signal to the subtractor at the same sampling rate as its minuend input signal.
In certain of these adaptive channel equalizers embodying the invention in a preferred form, the interpolation filter precedes the feedback FIR digital filter in their cascade connection, and the feedback FIR digital filter is operated at the same higher sampling rate as the precursor feed-forward FIR filter. Error signal for decision feedback is generated by differentially comparing the input signal of the rate-reduction filter with the output signal of the interpolation filter.
In other of these adaptive channel equalizers embodying the invention in a preferred form, the feedback FIR digital filter precedes the interpolation filter in their cascade connection, and the feedback FIR digital filter is operated at symbol rate. Error signal for decision feedback is generated by differentially comparing the input signal of the rate-reduction filter with the output signal of the quantizer as re-sampled by a further interpolation filter.